This invention relates generally to echo receiving systems and more particularly to visual display apparatus used in such systems.
As is known in the art, echo receiving systems, such as radar and sonar systems, have been used to detect the presence of objects producing echo returns from transmitted pulses of sonic or electromagnetic radiation. In order to visually present the information in the echo return for suitable operator interpretation, a visual display system is typically provided. One type of display is a Plan Position Indicator (PPI) wherein an intensity-modulated circular map-like display on which echo signals produced by reflecting objects are shown with range and azimuth angle displayed in polar (rho-theta) coordinates. Thus, in a radar system, in response to each one of a series of transmitted pulses, a radar return signal is produced at the output of a radar receiver. The return signal is made up of a sequence of analog target returns, or range sweep, in which the time of occurrence with respect to the transmitted pulse is a measure of the target distance, or range R, and the antenna direction at the moment of transmission provides the target azimuth, .theta.. Thus, as the transmitting antenna is scanned in azimuth, transmitted pulses are sent out with the result that a plurality of range sweeps of echo returns are produced, each one of such sweeps being at a correspondingly different azimuth angle. Thus, as the antenna scans in azimuth, echo returns from objects at various ranges at each azimuth are displayed on the display screen, typically a cathode ray tube (CRT). As described in Chapter 9 of Introduction to Radar Systems (Second Edition) by Merrill I. Skolnick, McGraw Hill Book Company (1980), the decay of the visual information displayed on the CRT should be long enough to allow the operator not to miss target detections, yet short enough not to allow the information painted on one scan to interfere with new information entered from the succeeding scan. However, there is usually not sufficient flexibility available to the CRT designer to always obtain the desired phosphor decay characteristics. The brilliance of the initial "flash" from the CRT phosphor may be high, but the afterglow is dim so that it is often necessary to carefully control the color and the intensity of the ambient lighting to achieve optimum seeing conditions. The conventional CRT usually requires a darkened room or the use of a viewing hood by the operator. Thus, as is known in the art, PPI type displays operate with relatively low intensity display screens and thus typically must operate under low level lighting conditions. There are applications where it is not practical to use a CRT display which requires a darkened environment such as in the cockpit of an aircraft or an air field control tower. In order to provide a rho-theta map-like display of a scanned region on a higher intensity display screen, such as a rectangular raster-type, TV, CRT display, a scan converter is generally required. In a simple form, the output of a normal PPI display is read out by a television camera in a rectangular row-column raster scan and is then displayed on a conventional TV raster monitor. The conversion of the radial rho (i.e. range)-theta (i.e. azimuth) raster of the PPI to the rectangular raster of the TV monitor represents a "scan conversion". The PPI display may be read out repetitively to produce a bright flicker-free raster display. The display is bright not only because the stored information is displayed at a sufficiently high repetition rate to appear continuous, but also because TV monitors have phosphor characteristics which result in such monitors being inherently brighter than the conventional PPI display. On the other hand, such type of scan conversion is expensive, bulky, generally has inadequate resolution, and not readily compatible with modern solid state systems. Thus, while it is desirable to retain the brightness characteristic of a TV type monitor, the use of a television camera tube is impractical.
Other types of scan converters which have been suggested include digital processing circuitry for first converting the analog echo returns into corresponding digital data. Again, the analog returns are in a rho-theta polar coordinate system. A digital scan conversion process is used to distribute the digitized data obtained in rho-theta coordinates into corresponding data in x-y rectangular coordinates so that the same map may be obtained after the x-y data is read out in a rowcolumn rectangular raster format on a TV screen monitor. Thus, the PPI data acquired in polar coordinates is transformed to data in Cartesian (rectangular) coordinates and such converted data is stored in a display memory usually referred to as a bit image memory (BIM). Each x-y location on the monitor corresponds to a specific x-y position in the memory. The location of radar video expressed in polar coordinates is transformed to Cartesian coordinates by the scan converter and determines the memory address at which to write the corresponding encoded video. As noted above, the number of memory cells in the BIM to be addressed corresponds to, on a one-to-one basis, the number of pixels, or display elements on the monitor. Thus, a 1024 line raster with 1024 pixels in the line direction requires a memory address capability of one Mega bit, while several parallel memory planes are required to present the information in colors or at different brightness levels. The appearance of the rectangular raster display, obtained by sequentially reading out successive locations of the memory, is presented to the operator slightly different from the conventional analog PPI. More particularly, whereas on the conventional analog PPI, unwanted signals, i.e. noise and clutter, usually returns of short time duration (such as sea clutter in a marine radar system), appear as relatively weak intensity signals due to the decay characteristics of the CRT screen, the rectangular raster monitor will show these unwanted signals at full strength for the duration of a scan; that is, for a full antenna revolution, typically a few seconds. On the other hand, desired signals, (i.e. targets which are characterized by returns which exist for relatively long time durations) may not appear on the display for a particular scan if during such particular scan there is a fade (or lapse) in the target return. This is because, in such case, a zero level intensity would have been stored in the memory for this "return" and will not be updated until the next antenna scan. Hence a null will appear on the monitor for the entire antenna scan unlike an analog PPI where the display of a target over a relatively long period of time (i.e. several scan cycles) increases (i.e. builds-up) the intensity on the screen and a fade (or lapse) for a short period of time will not cause a null on the next scan but merely will appear as reduced intensity because of the integration, and persistence, characteristic of the screen. More specifically, the following PPI display characteristics, to some degree realized in the analog PPI are desirable: (1) build up of target echo returns over one or more scans; (2) decay of targets over several scans; and, (3) trails on moving targets. The properties of the PPI CRT phosphors determine the extent to which a mix of desirable characteristics can be achieved at a still acceptable light output level on the PPI CRT. Trails, for instance, require a long decay time. Desirable fast target build-up and slow decay led to the use of two layer phosphors for PPI applications, however, at a less than desirable light output. In general then, each application demands its own compromise in PPI CRT phosphor selection.